This invention relates to the control of a motor vehicle drivetrain including an electrical machine, and more particularly to a control of the electrical machine for suppressing drivetrain oscillations referred to as surge.
It is well known that the combined components of a motor vehicle drivetrain (driveshaft, differential gearset, propshafts, suspension, tires, etc.) exhibit a characteristic torsional natural frequency, and that such components can be excited in a resonant mode, due to driver requested power changes, transmission shifting, irregular engine combustion or road disturbances. This phenomenon is sometimes referred to as surge or chuggle, and typically occurs in a frequency range of 1 Hz to 12 Hz, depending on the combined stiffness of the various drivetrain components. Unfortunately, the tendency of a vehicle to experience surge is significantly increased in high-efficiency operating modes, such as when the engine is operated at low speed and high load.
For obvious reasons, drivetrain surge is highly undesirable, and various approaches, both mechanical and electrical, have been proposed for mitigating its effects. See, for example, the U.S. Pat. No. 5,185,543 to Tebbe and U.S. Pat. No. 5,730,094 to Morris. In Tebbe ""543, a torsional vibration damper is coupled to a motor vehicle drivetrain; this tends to change the natural frequency of the drivetrain, making it less subject to resonant ringing. In Morris ""094, on the other hand, the field current of an engine-driven alternator is controlled to actively dampen a detected oscillation of the drivetrain by presenting a variable load torque to the engine. Morris ""094 also discusses active control of engine spark timing for the same purpose, as do the U.S. Pat. No. 5,669,354 to Morris and U.S. Pat. No. 5,573,474 to Marsh et al.
A common thread in the various electrical approaches mentioned above is that the suppression control is triggered into operation when the engine speed deviates from a steady value by more than a certain amount. For example, in the ""543 patent, the active damping control is initiated when the measured engine speed exceeds a reference engine speed by more than a dead band threshold such as xc2x15RPM. However, the engine speed is a complex waveform containing a variety of high and low frequency components, many of which cannot be perceived by the vehicle occupants and, therefore, need not be suppressed. Furthermore, the frequency response of the electrical control may severely limit its ability to actively dampen or cancel many of the detected frequency components.
The present invention is directed to a surge suppression control for a motor vehicle drivetrain, having an improved method of detecting and responding to drivetrain oscillation at the natural frequency of the drivetrain. According to the invention, the drivetrain natural frequency is determined based on the current speed ratio of a transmission coupling the drivetrain to the engine, and is used to define the center frequency of a band-pass filter responsive to engine speed. The filter output represents the AC component of the engine speed in a range of frequencies centered on the driveline natural frequency, and such component is differentiated to form an acceleration component in phase synchronism with the torque causing the detected resonance. An electric machine mechanically coupled to the engine is energized in accordance with the inverse of the acceleration component to actively cancel the detected resonance whenever the acceleration component exceeds a noise threshold that also is adjusted based on the estimated natural frequency of the drivetrain.